Polyethylene is a thermoplastic material that is created through polymerization of ethylene monomer, and which is used in the manufacture of a wide variety of consumer products, including packaging, pipe extrusion, wire and cable sheathing and insulation, and many other products. Because ethylene has no substituent groups to influence the stability of the propagation head of the growing polymer chain, polymers of varying degrees of branching can be produced through radical polymerization, anionic addition polymerization, ion coordination polymerization or cationic addition polymerization. Today one of the most common methods of preparing highly desirable linear (high density) polyethylene involves contacting ethylene with a Ziegler-Natta catalyst system that includes a transition metal catalyst such as TiCl4 and an organo-compound of a non-transition metal of Groups IA to IIIA of the Periodic Table of the Elements, particularly organo-aluminium compounds.
Polypropylene is another thermoplastic polymer that is widely used in the manufacturing of a variety of products, including housings and parts for small and large appliances, disposable containers, food packaging, ropes, textiles and plastic automobile parts, and many more. It is chemically synthesized by the catalyzed polymerization of propylene monomer. Polypropylene is most often produced as a stereospecific polymer. Isotactic polypropylene has all the pendant methyl groups oriented either above or below the polymer chain. Any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer. Most commercially available polypropylene is made with titanium chloride catalysts to produce substantially isotactic polypropylene, which is highly desirable for making a number of products that require a strong polymer.
Ziegler-Natta catalysts are stereospecific complexes that limit incoming monomers to a specific orientation, only adding them to the polymer chain if they are oriented in a specific direction, to produce isotactic (unbranched) polymers. Because the organo-compounds of transition metals are useful polymerization catalysts only when supported, they are supported on a suitable matrix material such as alumina, silica, or magnesia. Conventional Ziegler-Natta catalysts are stereospecific complexes formed from a halide of a transition metal, such as titanium, chromium or vanadium with a metal hydride and/or metal alkyl, typically an organoaluminum compound such as an alkylaluminum compound, for example, triethylaluminum (TEAL), trimethyl aluminum (TMA) or triisobutyl aluminum (TIBAL), as a co-catalyst. Both liquid phase slurry (suspension) polymerization and gas phase polymerization have been catalyzed using Ziegler-Natta catalysts. Although polymerization rates increase with temperature, reaction temperatures above 70-100° C. seldom are employed because high temperatures result in loss of stereospecificity as well as lowered polymerization rates as a result of the decreased stability of the initiator. In many polyolefin manufacturing processes today metallocene based catalysts are replacing some Ziegler-Natta catalysts.
Other transition metal catalysts that polymerize ethylene are based on the oxides of chromium or molybdenum. Other transition metal catalyst systems include the organo-compounds of transition metals with π-allyl, cyclopentadienyl, norbornyl, benzyl, and arene groups and also compounds including groups of the type exemplified by the neopentyl and substituted silylmethyl compounds. Catalysts that promote branching of the polymer are employed when a low-density polyethylene is sought.
In a typical liquid phase slurry (suspension) polymerization process ethylene or propylene monomer is dissolved in an organic reaction medium and then contacted with a particulate catalyst. The polyethylene or polypropylene that is formed is also dissolved in the organic medium, which can become quite viscous. Although polymerization rates increase with temperature, reaction temperatures above 70-100° C. seldom are employed because high temperatures result in loss of stereospecificity as well as lowered polymerization rates as a result of the decreased stability of the catalyst.
At the present time, solution polymerization is generally considered to be limited to production of low molecular weight polyethylene and polypropylene. Existing processes and production facilities for producing these polymers are typically subject to various constraints including mass flow limitations, product yield, plant size and energy consumption. Accordingly, there is continued interest in the development of ways to improve the selectivity and yield of polymers from catalyzed polymerization of ethylene and propylene monomers.